Sugar Transport and Metal Binding in Yeast

The uptake of sugars by yeast can be separated into two classes. The first involves the uptake of sorbose or galactose by starved cells, and the uptake of glucose by iodoacetate-poisoned cells. These uptakes do not involve any changes in Ni⁺⁺- or Co⁺⁺-binding by the cell surface, are not inhibited by Ni⁺⁺, are inhibited by UO₂⁺⁺ in relatively high concentrations, are characterized by high Michaelis constants and low maximal rates and by a final equilibrium distribution of the sugars. The second involves the uptake of glucose in unpoisoned cells and galactose in induced cells. These uptakes are characterized by a reduction of Ni⁺⁺- and Co⁺⁺-binding, by a partial inhibition by Ni⁺⁺, by an inhibition with UO₂⁺⁺ in relatively low concentrations, and by a low Km and a high Vm. In the case of galactose in induced cells, previous studies demonstrate that the sugar is accumulated against a concentration gradient. It is suggested that the first class of uptakes involves a "facilitated diffusion" via a relatively non-specific carrier system, but the second represents an "uphill" transport involving the highly specific carriers, and phosphoryl groups (cation-binding sites) of the outer surface of the cell membrane.